Generally, when completing a subterranean well for the production of fluids, minerals, or gases from underground reservoirs, several types of tubulars are placed downhole as part of the drilling, exploration, and completions process. These tubulars can include casing, tubing, pipes, liners, and devices conveyed downhole by tubulars of various types. Each well is unique, so combinations of different tubulars may be lowered into a well for a multitude of purposes.
A subsurface or subterranean well transits one or more formations. The formation is a body of rock or strata that contains one or more compositions. The formation is treated as a continuous body. Within the formation hydrocarbon deposits may exist. Typically a wellbore will be drilled from a surface location, placing a hole into a formation of interest. Completion equipment will be put into place, including casing, tubing, and other downhole equipment as needed. Perforating the casing and the formation with a perforating gun is a well known method in the art for accessing hydrocarbon deposits within a formation from a wellbore.
Explosively perforating the formation using a shaped charge is a widely known method for completing an oil well. A shaped charge is a term of art for a device that when detonated generates a focused explosive output. This is achieved in part by the geometry of the explosive in conjunction with a liner in the explosive material. Generally, a shaped charge includes a metal case that contains an explosive material with a concave shape, which has a thin metal liner on the inner surface. Many materials are used for the liner, some of the more common metals include brass, copper, tungsen, and lead. When the explosive detonates the liner metal is compressed into a super-heated, super pressurized jet that can penetrate metal, concrete, and rock.
A perforating gun has a gun body. The gun body typically is composed of metal and is cylindrical in shape. The gun body will have one or more scallops machined out of its surface. A scallop is a thin spot on the gun body, usually circular in shape. Each scallop is designed to align with a corresponding shape charge contained within the gun body. The scallop allows for a uniform thickness of metal that is at a ninety degree angle with respect to the plasma jet that exits an exploding shape charge. The scallop allows for less explosive energy needed to exit the gun body and for a more predictable trajectory of the plasma jet into the formation being perforated. Moreover, the scallops reduce the impact burrs created during detonation that may interfere with moving the gun or retrieving the gun. However, since the shape charges are typically located on a separate charge holder located within the gun body, it is difficult to perfectly align each scallop with its corresponding shape charge.
Within a typical gun tube is a charge holder, which is a tube that is designed to hold the actual shape charges. The charge holder will contain cutouts called charge holes where the shape charges will be placed. The alignment of the charge holder with the gun tube is important in controlling the position of the shape charges when they detonate. Error in the orientation of the shape charge with respect to the scallop can cause the explosive jet to miss its intended target within the formation.
A charge tube may have end fittings attached at either end. The end fittings provide for restricting the movement of the charge tube with respect to the gun body. The current method for aligning the charge holder with the gun tube involves using an alignment end with a slot and bolt design on the gun tube. The alignment end contains a bolt head or some other protrusion. The bolt is lined up with the first shape charge. The charge holder with the alignment end is then placed inside the gun body. The gun body has a slot cut out of the inner surface. The slot is aligned with the first scallop on the gun barrel, which also happens to be the closest scallop to the alignment end. The bolt on the alignment end will normally interfere with the threads or the inner surface of the gun body if there is no slot. Thereby preventing the charge holder from being placed in the gun body. However, the slot cut will allow the charge holder to be placed into the gun body so long as the bolt head is properly aligned with the slot cut. The slot cut on the gun body is aligned with the first scallop on the outside of the gun body.
This slot and bolt design is robust and simple in its execution. The problem with this design is that the slot has to be machined through the threads. Moreover, the length of the slot cut is limited by the machining capabilities as the slot cannot be milled into the inner gun body for more than a few inches from the end of the gun body. The bolt cannot be on the bottom of the gun charge because then the gun charge cannot be dropped into the gun tube from the top. This would require a slot machined into the side of the gun tube for the entire length of the gun tube, which is difficult and impractical. Therefore, this method cannot be used to align both ends of the charge holder with the gun tube. In some cases, such as with long perforating guns, the charges distant from the alignment end can drift out of alignment with the scallops, thereby reducing charge efficiency. Further, this method has certain manufacturing and installation limitations.